The G-Virus, like the T-Virus, is a man-made disease from the Resident Evil Video Games.
The G-Virus was created by William Birkin in an Umbrella Corporation lab. He intended to sell his findings to the government, when Umbrella caught up to him and nearly killed him. he tried to avoid death by injecting the virus directly into himself, destroying most of his humanity, but constantly repairing his cells. Somehow, the virus spread to rats, who infected human beings and turning them into shambling, flesh-hungry zombies.